Thanks for the response. Your response has given me a clue how to enbale the link. Although with my previous profile I was no able to see the PegaFW:SocialAdmin role. Now I have created my application on top of CSSocial:07.21 which is built on top of CustomerService:07.21. I can accees the Social Engagement Landing Page now.
Social engagement feature in CS has undergone major architecture changes in CS 7.21. Please refer to the section "Pega Customer Service Social Engagement Implementation" in the guide "https://docs.pega.com/pega-customer-service-721-implementation-guide" for configuring the feature. Let us know if you are still facing issues with the configuration.